Composite sounding for MIMO beamforming in a wireless home network

ABSTRACT

A WAP including: a sounding mode module, a sounding matrix generator, a sounding aggregator, a gain normalizer and a beamforming expansion module. The sounding mode module determines whether a number of communication streams supported by the WAP matches the number of streams contained in a sounding response from a station, and initiates a composite set of soundings when those capabilities do not match. The sounding matrix generator generates linearly independent spatial mapping matrices each associated with a corresponding one of the set of composite soundings and at least one reference SMM. The sounding aggregator aggregates partial sounding feedback matrices received from the targeted station node in response to the composite soundings. The gain normalizer normalizes the partial sounding feedback matrices utilizing the reference SMM. The beamforming expansion module expands the aggregated sounding feedback matrices into a full beamforming matrix for spatially mapping downlink communications.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of co-pending U.S. Utilitypatent application Ser. No. 14/701,495 Filed Apr. 30, 2015 and entitled“Composite Sounding for MIMO Beamforming in a Wireless Home Network”which in turn claims the benefit of prior filed Provisional ApplicationNo. 61/986,365 filed on Apr. 30, 2014 entitled “Composite Sounding forSystems with High Number of Transmit Antenna” and 62/000,940 filed onMay 20, 2014 entitled “Composite Sounding for Single-Antenna ClientStas” all of which are incorporated herein by reference in theirentirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The field of the present invention relates in general to wireless localarea networks including wireless access points (WAP) and wirelessstations and sounding methods therefore.

2. Description of the Related Art

Home and office networks, a.k.a. wireless local area networks (WLAN) areestablished using a device called a Wireless Access Point (WAP). The WAPmay include a router. The WAP wirelessly couples all the devices of thehome network, e.g. wireless stations such as: computers, printers,televisions, digital video (DVD) players, security cameras and smokedetectors to one another and to the Cable or Subscriber Line throughwhich Internet, video, and television is delivered to the home. MostWAPs implement the IEEE 802.11 standard which is a contention basedstandard for handling communications among multiple competing devicesfor a shared wireless communication medium on a selected one of aplurality of communication channels. The frequency range of eachcommunication channel is specified in the corresponding one of the IEEE802.11 protocols being implemented, e.g. “a”, “b”, “g”, “n”, “ac”, “ad”.Communications follow a hub and spoke model with a WAP at the hub andthe spokes corresponding to the wireless links to each ‘client’ device.

After selection of a single communication channel for the associatedhome network, access to the shared communication channel relies on amultiple access methodology identified as Collision Sense MultipleAccess (CSMA). CSMA is a distributed random access methodology firstintroduced for home wired networks such as Ethernet for sharing a singlecommunication medium, by having a contending communication link back offand retry access to the line if a collision is detected, i.e. if thewireless medium is in use.

Communications on the single communication medium are identified as“simplex” meaning, one communication stream from a single source node toone or more target nodes at one time, with all remaining nodes capableof “listening” to the subject transmission. Starting with the IEEE802.11ac standard and specifically ‘Wave 2’ thereof, discretecommunications to more than one target node at the same time may takeplace using what is called Multi-User (MU) multiple-inputmultiple-output (MIMO) capability of the WAP. MU capabilities were addedto the standard to enable the WAP to communicate with multiple singleantenna single stream devices concurrently, thereby increasing the timeavailable for discrete MIMO video links to wireless HDTVs, computerstablets and other high throughput wireless devices the communicationcapabilities of which rival those of the WAP.

In order to characterize the communication channel between the WAP andeach station a sounding is conducted. An explicit sounding consists ofthe transmission of a known sequence from the WAP to each associatedstation, followed by a sounding response from the station characterizingthe communication channel between the WAP and itself. The WAP uses thesounding response to focus its antennas in a manner which improveseither or both signal strength at the station or downlink throughputthereto.

What is needed are improved methods for sounding each communication linkbetween the WAP and its associated stations.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for a wirelessaccess point (WAP) apparatus to enhance communications with targetstations that do not support sounding feedback which matches thecapability of the WAP.

In an embodiment of the invention a wireless access point (WAP)apparatus having a plurality of antennas and supporting multiple-inputmultiple-output (MIMO) wireless communications with associated stationnodes on a selected one of a plurality of orthogonal frequency divisionmultiplexed (OFDM) communication channels each including a plurality oftones is disclosed. The WAP includes: a sounding mode module, a soundingmatrix generator, a sounding aggregator, a gain normalizer and abeamforming expansion module. The sounding mode module is configured todetermine whether a number of communication streams supported by the WAPmatches the number of communication streams contained in a soundingresponse from at least one targeted station node among the associatedstation nodes, and to initiate either a single sounding or a compositeset of an integer “N” soundings of the communication channel between theWAP and the at least one targeted station node based on an affirmativeand a negative match determination respectively. The sounding matrixgenerator is configured to generate “N” linearly independent (LI)spatial mapping matrices (SMM) each associated with a corresponding oneof the soundings in the composite set of soundings from the WAP,together with at least one reference SMM for spatially mapping at leastone reference tone across all of the soundings in the composite set ofsoundings, responsive to a negative match determination by the soundingmode module. The sounding aggregator is configured to aggregate aplurality of partial sounding feedback matrices received from the atleast one targeted station node in response to the composite soundingsgenerated by the sounding matrix generator. The gain normalizernormalizes the partial sounding feedback matrices aggregated by thesounding aggregator utilizing the reference SMM. The beamformingexpansion module is configured to expand the plurality of partialsounding feedback matrices aggregated by the sounding aggregator into afull beamforming matrix for spatially mapping downlink communicationsbetween the WAP and the at least one targeted station node; therebyenhancing the communication capabilities of the WAP.

The invention may be implemented in hardware, firmware or software.

Associated methods and computer readable media containing programinstructions are also claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIGS. 1A-D are respectively Prior Art downlink sounding andcommunication system views, and sounding packet timing and packetstructure diagrams of a communication link between a Wireless AccessPoint (WAP) and a station with constrained sounding capabilities;

FIGS. 2A-D are respectively downlink sounding and communication systemviews, and sounding packet timing and packet structure diagrams of acommunication link between a Wireless Access Point (WAP) and a stationwith constrained sounding capabilities in accordance with an embodimentof the invention;

FIGS. 3A-D are system views of a downlink composite sounding set inaccordance with an embodiment of the invention;

FIGS. 4A-B are system views of a multi-user (MU) multiple-inputmultiple-output (MIMO) downlink to stations with constrained soundingcapabilities using a prior art sounding (FIG. 4A) and a composite set ofsoundings in accordance with an embodiment of the invention (FIG. 4B)respectively;

FIGS. 5A-C are signal diagrams of a MIMO downlink to a station withconstrained sounding capabilities: without beamforming (FIG. 5A), withbeamforming resulting from a prior art sounding (FIG. 5B) and withbeamforming resulting from a composite sounding in accordance with anembodiment of the invention (FIG. 5C);

FIG. 6 is a hardware block diagram of a MU-MIMO WAP with support foreither single sounding or composite soundings depending on the relativecapabilities of the WAP and the target station(s); and

FIG. 7 is a process flow diagram of processes associated with enhancedsounding capabilities of a MU MIMO WAP in accordance with an embodimentof the current invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention provides a method and apparatus for increasingdownlink throughput from a multiple-input multiple-output (MIMO) WAP anda station.

FIGS. 1A-D are respectively Prior Art downlink sounding andcommunication system views, and sounding packet timing and packetstructure diagrams of a communication link between a Wireless AccessPoint (WAP) and a station with constrained sounding capabilities.

FIG. 1A is a system diagram showing a sounding by a Prior Art WAP 100 ofa downlink channel between it and a target station 120. The WAP, a.k.a.a 4×4 WAP, has 4 antennas and supports up to 4 streams of downlink anduplink communications. The WAP has a baseband section 102 and a radiofrequency (RF) section 110 coupled to 4 multiple-input multiple-output(MIMO) antennas 112. The baseband transmit portion is shown and thecorresponding receive portion is not. A spatial mapper 104 maps up tofour communication streams onto the communication chains associated eachof the antenna. The mapping includes changes to the phase and amplitudeof each stream to focus, a.k.a. beamform, the downlink communications tothe station. The output of the spatial mapper is coupled to the inputbins 106A-D of the inverse discrete Fourier Transform (IDFT) modules108A-D which transform the transmitted communication from frequency tothe time domain. These in turn couple through associated RF componentswith a corresponding one of the four antenna 112. The station 120 isidentified as a 2×2 device having two antenna and supporting up to 2streams of downlink and uplink communications. The station has abaseband section 122 and a radio frequency (RF) section 130 coupled to 2multiple-input multiple-output (MIMO) antennas 132. The baseband receiveportion is shown and the corresponding transmit portion is not. Thediscrete Fourier Transform (DFT) modules 128A-B are coupled to acorresponding one of the two antenna 132, to transform receivedcommunications from the time to the frequency domain. The output bins126A-B of the DFT modules are coupled to the input of the equalizer 124.The sounding feedback capabilities of the station 120 do not match theWAPs capabilities in terms of number of streams and antennas. The WAPcould handle up to 4 streams on a downlink sounding, but can't do sobecause the station is only capable of providing sounding feedback forup to two streams. This Prior Art sounding is limited to a singlespatial mapping matrix (SMM) 142 for the single channel/link sounding.The SMM maps the proscribed sounding bits and phases redundantly ontothe MIMO antenna array, i.e. 2 pairs of antenna each have the samesounding signal, phase and amplitude. The sounding is said to bepartial, because it does not include unique signals on each of theantenna and thus does not allow channel characterization of all possiblepaths 140B (dotted lines) between the transmit and receive antennas ofthe WAP and station respectively. The station 120 receives this partialsounding 140A and determines the two stream beamforming matrix 144 whichwill help to focus the subsequent downlink communications from the WAP.

As shown in FIG. 1B the Prior Art WAP 100 uses the single beamformingmatrix 144 provided by the station in its sounding response as an inputto the spatial mapper 104, to steer/focus/beamform 148 the downlinkcommunications from its MIMO antenna array. Unfortunately, thebeamforming signal strength or focus is limited by the lack of acomplete characterization of all the possible paths between Transmit andReceive antennas during the single sounding. In other words, with asingle two stream sounding response provided by the station 120 the WAPis unable to fully characterize the downlink communication channel, thusdegrading its beamforming capabilities.

FIG. 1C is a Prior Art detailed explicit sounding timing diagram showinga detailed view of a representative Prior Art sounding as shown in FIG.1A. An Explicit sounding of the link channel between the WAP 100 andstation 120 is shown. The packets 150, 152, 144 are all associated withthe sounding. All packets including those associated with the sounding,include a header portion shown in crosshatch. Following the sounding,communications resume and downlink communication of user data is sent onthe link(s) that have been sounded. These user data packet(s) are sentusing the associated beamforming matrix determined during the precedingsounding.

The explicit sounding provided for in the IEEE 802.11 ac standard allowsthe receiver to assist the transmitter to steer subsequent user datacommunications toward the receiver using the beamforming matrix providedby the receiver in response to the explicit link channel soundinginitiated by the transmitter. An explicit sounding may be initiated by aWAP or a station. In the example shown the WAP 102 initiates thesounding by sending at time to sends a null data packet announcement(NDPA) frame 150. The NDPA identifies the WAP and the target recipientstation(s) for the sounding. Next at time t₁ a null data packet (NDP)152 is sent by the WAP. This packet like all the other packetsassociated with the sounding contains no user data rather the header ofthe packet contains a ubiquitous preamble field, which in the case ofthe IEEE 802.11ac standard is identified as the VHT-LTF field 170 shownin FIG. 1D. The VHT-LTF field a.k.a. channel estimation or soundingfield, contains a long training sequence used for MIMO channelestimation by the receiver. Each recipient device then determines thecorresponding beamforming matrix required to adjust the phase andamplitude of subsequent MIMO transmissions by the WAP so as to improvethe received signal strength at the receiving station. The target e.g.station 120 then responds at time t₂ with the beamforming feedbackpacket 144 containing channel state information (CSI) including thebeamforming matrix “V”. In the example shown, the “V” matrix is a 2×2matrix. If the receiving station is IEEE 802.11ac compliant the feedbackis in the form of the actual unitary beamforming matrix V and the pertone diagonal matrix Σ which is directly related to the per tone signalto noise ratio (SNR).

FIG. 1D is a packet diagram of a WLAN packets including the soundingfield. All WLAN packets whether associated with communicating a soundingor with the communication of user data include a ubiquitous headerportion. All WLAN packet headers include various preamble fields withknown sequences which allow the receiving station to synchronizereception with packet boundaries and to determine the received channel.What makes a sounding packet a sounding packet is not the sounding fieldin the header, rather the NDPA payload instructions which identify thereceiving stations which are requested to share their channel analysiswith the transmitter so as to improve its subsequent communications.FIG. 1D shows such a packet 160 and the corresponding symbol interval(SI) required to transmit each field thereof. The header 162A includes alegacy portion containing the L-STF, L-LTF and L-SIG fields and a veryhigh throughput portion containing the VHT-SIG-A, VHT-STF, VHT-LTF andVHT-SIG-B fields. The legacy (L), long (LTF) and short (STF) trainingand signal (SIG) 164 fields are compatible with stations supporting onlythe IEEE 802.11n or earlier standards. The remaining signal and trainingfields are intended only for very high throughput, e.g. IEEE 802.11accompliant devices. The VHT-SIG-A field 166 contains information on themodulation and coding scheme (MCS) and number of streams of thesounding. The VHT-STF field 168 is used for automatic gain control(AGC). The VHT-LTF field 170, a.k.a. channel estimation or soundingfield, contains a long training sequence used for MIMO channelestimation by the receiver.

FIGS. 2A-D are respectively downlink sounding and communication systemviews, and sounding packet timing and packet structure diagrams of acommunication link between a Wireless Access Point (WAP) and a stationwith constrained sounding capabilities in accordance with an embodimentof the invention;

FIG. 2A is a system diagram showing a composite sounding by WAP 200 of adownlink channel between it and a target station 120. The WAP, a.k.a. a4×4 WAP, has 4 antennas and supports up to 4 streams of downlink anduplink communications. It supports composite soundings when required tofully characterize a communication channel. The WAP has a basebandsection 202 and a radio frequency (RF) section 210 coupled to 4multiple-input multiple-output (MIMO) antennas 212. The basebandtransmit portion is shown and the corresponding receive portion is not.A spatial mapper 204 maps up to four communication streams onto thecommunication chains associated each of the antenna. The mappingincludes changes to the phase and amplitude of each stream to focus,a.k.a. beamform, the downlink communications to the station. The outputof the spatial mapper is coupled to the input bins 206A-D of the inversediscrete Fourier Transform (IDFT) modules 208A-D which transform thetransmitted communication from frequency to the time domain. These inturn couple through associated RF components with a corresponding one ofthe four antenna 212. The station 120 is identified as a 2×2 devicehaving two antennae and supporting up to 2 streams of downlink anduplink communications. The station has a baseband section 122 and aradio frequency (RF) section 130 coupled to 2 multiple-inputmultiple-output (MIMO) antennas 132. The baseband receive portion isshown and the corresponding transmit portion is not. The discreteFourier Transform (DFT) modules 128A-B are coupled to a correspondingone of the two antenna 132, to transform received communications fromthe time to the frequency domain. The output bins 126A-B of the DFTmodules are coupled to the input of the equalizer 124. The soundingfeedback capabilities of the station 120 do not match the WAPscapabilities in terms of number of streams and antennas. The WAP detectsthis sounding capability mismatch and determines the number of compositesoundings and the linearly independent (LI) spatial mapping matrices(SMM) 242 associated with each that will allow for full characterizationof the downlink communication channel between the WAP and the station.The WAP sends out each of the Linearly Independent partial soundings240A-D using the LI SMM, and receives in response to each a singlepartial sounding response, the aggregate of which is the set 244A-D ofpartial sounding responses.

As shown in FIG. 2B the WAP 200 aggregates the set of partial soundingresponses 244A-D and expands them into a full beamforming matrix 246 forsteering/focusing/beamforming subsequent downlink communications to theWAP. This fully dimensioned beamforming matrix maximizes the RF signalstrength 248 from the MIMO antenna array at the antennas 132 of thereceiving station 120. This corresponds to a 3-6 dB improvement insignal strength as opposed to the single sounding of the prior art WAPshown in FIGS. 1A-B.

FIG. 2C is a detailed explicit sounding timing diagram showing arepresentative composite sounding in accordance with an embodiment ofthe invention shown in FIG. 2A. An Explicit sounding of the linkchannels between the WAP 200 and station 120 are shown. The packets 250,252, 244 are all associated with the composite sounding. All packetsincluding those associated with the sounding, include a header portionshown in crosshatch. Following the sounding, communications resume, anduser data is sent on the link(s) that have been sounded. These user datapacket(s) are sent using the associated beamforming matrix determinedduring the preceding composite sounding.

The explicit sounding provided for in the IEEE 802.11 ac standard allowsthe receiver to assist the transmitter to steer subsequent user datacommunications toward the receiver using the beamforming matrix providedby the receiver in response to the explicit link channel soundinginitiated by the transmitter. An explicit sounding may be initiated by aWAP or a station. In the example shown the WAP 200 initiates thesounding by sending at time to a null data packet announcement (NDPA)frame 250. The NDPA identifies the WAP and the target recipientstation(s) for the sounding. Where more than one station is a targetrecipient, the order in which the recipient stations are listed controlsthe order of their response. Next at time t₁ a null data packet (NDP)252A is sent by the WAP. This packet like all the other packetsassociated with the sounding contains no user data rather the header ofthe packet contains a ubiquitous preamble field, which in the case ofthe IEEE 802.11ac standard is identified as the VHT-LTF field 270 shownin FIG. 2D. The VHT-LTF field a.k.a. channel estimation or soundingfield, contains a long training sequence used for MIMO channelestimation by the receiver. Each IEEE 802.11ac compliant recipientdevice then determines the corresponding beam steering/beamformingmatrix, i.e. “V” and “SNR”, required to adjust the phase and amplitudeof subsequent MIMO transmissions by the WAP so as to maximize thereceived signal strength at the receiving station. The target e.g.station 120 then responds at time t₂ with the beamforming feedbackpacket 244A containing channel state information (CSI). If the receivingstation is IEEE 802.11 ac compliant the feedback is in the form of theactual unitary beam steering matrix V and the per tone diagonal matrixSNR. Any remaining stations targeted by the initial sounding, respondwith the beam steering matrix for their own link when asked to do so bythe WAP. The WAP in this embodiment of the invention has determinedbased on the mismatch between the # of streams supported by the WAP andthe number of streams in sounding feedback from the station thatcomposite soundings are required. It then sends three subsequentsoundings 252B-D each prompting a corresponding one of sounding feedbackpacket responses 244B-D. These composite soundings each sent withlinearly independent spatial mapping allow the WAP to expand the channelcharacterization to a full beamforming matrix in which all paths betweenthe MIMO transmit and receive antenna are characterized. Alternatively,the expanded sounding information that is obtained from the set ofpartial sounding feedbacks can be used to assist in calibration of thetransmit and receive paths of the WAP.

FIG. 2D is a packet diagram of a WLAN packets including the soundingfield. All WLAN packets whether associated with communicating a soundingor with the communication of user data include a ubiquitous headerportion. All WLAN packet headers include various preamble fields withknown sequences which allow the receiving station to synchronizereception with packet boundaries and to determine the received channel.What makes a sounding packet a sounding packet is not the sounding fieldin the header, rather the NDPA payload instructions which identify thereceiving stations which are requested to share their channel analysiswith the transmitter so as to improve its subsequent communications.FIG. 1D shows such a packet 260 and the corresponding symbol interval(SI) required to transmit each field thereof. The header 262A includesthe legacy portion containing the L-STF, L-LTF and L-SIG fields and thevery high throughput portion containing the VHT-SIGA, VHT-STF, VHT-LTFand VHT-SIGB fields. The legacy (L), long (LTF) and short (STF) trainingand signal (SIG) 264 fields are compatible with stations supporting onlythe IEEE 802.11n or earlier standards. The remaining signal and trainingfields are intended only for very high throughput, e.g. IEEE 802.11accompliant devices. The VHT-SIGA field 266 contains information on themodulation and coding scheme (MCS) and number of streams of thesounding. The VHT-STF field 268 is used for automatic gain control(AGC). The VHT-LTF field 270, a.k.a. channel estimation or soundingfield, contains a long training sequence used for MIMO channelestimation by the receiver. The VHT-LTF field of each sounding packet isspatially mapped with an associated one of a set of linearly independentspatial mapping matrices as determined by the WAP.

FIGS. 3A-D are system views of a downlink composite sounding set inaccordance with an embodiment of the invention. Four soundings 340A-Deach transmitted using a corresponding one of four linearly independent(LI) spatial mapping matrices (SMM) 342A-D are used to fullycharacterize the downlink communication channel via the aggregate setmade up of the four individual partial sounding responses 344A-D. TheWAP 300 is an 8×8 device with 8 antennas 312A-H and up to 8 streams. Themapping of streams to antennas is handled by the spatial mapper 304. Thereceiving station 320 is a 2×2 station with 2 antenna 332 and up to 2communication streams. This station can provide sounding feedback on upto four sounded streams but can only support downlink communications ontwo. The station includes a baseband stage 322 and an RF stage 330. Thebaseband stage includes DFT modules 328A-B the output bins of which326A-B are coupled to the equalizer 324.

The WAP upon determining that the station's sounding feedback is limitedto fewer streams than can be supported by the WAP, initiates thesequential partial soundings shown in FIGS. 3A-3D. Soundings arecharacterized as partial if they explore less than all the streams fromthe WAP to the station being sounded. Each partial sounding is conductedon all the OFDM tones 351 of the communication channel being sounded.Within the set of OFDM tones 351, the WAP designates selected tone(s) asreference tones, e.g. tones 351D, 351H. If more than one reference toneis selected, they are typically evenly spaced apart among the remainingtones. The reference tones are used by the WAP to normalize anydifferences in the partial sounding responses 344A-D relative to oneanother, e.g. differences in phase and/or amplitude, before expansioninto a full beamforming matrix for spatially mapping subsequentcommunications of user data on the downlink from the WAP to the station.The tones 351D, 351H selected as reference tones by the WAP are notknown by or identified to the station. Rather they are used exclusivelyby the WAP to normalize any amplitude or phase differences resultingfrom the sequential soundings 340A-D, and exhibited in the partialsounding feedback responses 344A-D respectively. Reference tones havethe same indices, e.g. center frequency, across the successive partialsoundings. The reference tone(s) are spatially modulated with areference spatial mapping matrix (SMM), e.g. SMM 343. Where there ismore than one reference tone per partial sounding there may be more thanone reference SMM, however, for a reference tone of a given index, thesame reference SMM is used across all the successive partial soundings.The remaining tones are spatially modulated with linearly independentspatial mapping matrices 342A-D. In the example shown, the 1^(st)composite sounding shown in FIG. 3A spatially maps streams “a-d” ontoindividual ones of antennas 312A-D respectively. The 2^(nd) compositesounding shown in FIG. 3B spatially maps streams “a-d” onto individualones of antennas 312E-H respectively. The 3^(rd) composite soundingshown in FIG. 3C spatially maps each of streams “a-d” onto acorresponding pair of antenna, e.g. stream “a” onto antenna 312A, 312E;stream “b” onto antenna 312B, 312F; stream “c” onto antenna 312C, 312G;stream “d” onto antenna 312D, 312H; and includes a scalar to normalizetotal transmit power with that of the other soundings. The 4^(th)composite sounding shown in FIG. 3D is identical to the third with theexception of the signal on antennas 312E-G which are each rotated 90degrees.

The WAP aggregates the partial sounding feedback/responses 344A-D asthey are received. The WAP aggregates each partial sounding feedbackincluding partial sounding feedback matrices, e.g. V, SNR, and or “H”matrices for all tones including the reference tones. The WAP firstprocesses the sounding feedback matrices for the reference tones andexploits the fact that they are spatially mapped with a fixed referenceSMM. This allows the WAP to normalize differences in amplitude and orphase between successive soundings by adjusting for correspondingdifferences in the sounding feedback for those tones designated asreference tones and applying the resulting normalizing scale factors tothe sounding feedback matrices for all remaining tones in each set oftones associated with each sounding feedback. The WAP then expands theaggregated, normalized partial soundings into a full beamforming matrixfor subsequent communication with the station (MIMO) or with more thanone station (MU-MIMO).

Example: 8×8 WAP to STATION with 4 Stream Feedback

The 802.11ac standard allows for MIMO systems with up to eight antennas.Using eight antennas at the transmitter provides advantages forbeamforming and multi-user MIMO. In order to get the optimal benefit,the system also needs to have access to channel information, which isprovided by the receivers in response to “sounding frames”. However, theresponse capabilities of some receivers may not be sufficient fortransmitters with higher numbers of antennas (say, larger than four).Below, we describe a method to get channel information from “legacyreceivers” (i.e. receivers that provided only four-stream feedback) andcombine that information into channel information that is optimal foruse by an eight-antenna device.

Assume we have a transmitter with eight antennas. To get the bestperformance for beamforming or MU-MIMO, we need to get the as muchinformation about the channel as possible. Therefore, we'd like to havesounding information that allows us to individually control the eightdegrees of freedom represented by the transmit antennas. In practice,this means that the intended receiver has to provide eight-streamfeedback in response to an eight-stream sounding frame. A receiver couldtheoretically provide sounding feedback for eight transmit antennas,even if it's number of receive antennas is lower than eight (even as lowas one). In practice however, many clients choose to only process alower number of streams to limit the complexity of the receiverimplementation. This leaves us with the following problem statement:Given a transmitter with eight antennas and a receiver that is onlycapable of providing feedback to a four-stream sounding packet, can weobtain information that is equivalent to eight-stream feedback by anumber of sequential four-stream soundings? We explicitly assume fourstream feedback in the remainder of the analysis, although this is notstrictly inherent to the mechanism.

Sequential Soundings

The channel between an eight-antenna receiver and a receiver with N_(RX)antennas is described by an N_(RX)×8 matrix H_(N) _(RX) _(×8). Thismatrix can be decomposed using SVD as follows:H _(N) _(RX) _(×8) =U _(N) _(RX) _(×N) _(RX) ΣN _(N) _(RX) _(×8) V_(8×8) ⁺  (1)A receiver capable of processing an eight-stream sounding packet wouldprovide information that is equivalent to Σ_(N) _(RX) _(×8) and V_(8×8).This corresponds to the “MU-style” sounding feedback. Where the numberof communication streams supported by the WAP exceeds the number ofstreams for which a receiving station can characterize a channelsounding, the station's sounding feedback is characterized as partialsounding feedback, because it characterized less than all the streamssupported by the WAP. Either of two approaches may be utilized toovercome this limitation, allowing the full channel to be characterizedby aggregation of successive partial sounding feedback matrices. In thefirst of these approaches the partial sounding feedback matrices arepartial beamforming matrices: e.g. both V and Σ. In the second of theseapproaches the partial sounding feedback matrices are partial channelmatrices: e.g. “H” matrices.

Partial Beamforming Matrices Sounding Feedback:

The first approach to reconstructing a fully dimensioned beamformingmatrix for the WAP relies on the availability of both Σ and V matrices.The challenge is to derive essentially the same information whilestaying within the limitations of the receiver (i.e. working withfour-stream feedback only). If the receiver can only process M=4streams, we map the M streams to the eight antennas using a SpatialMapping matrix Q with dimensions 8×M. The resultant channel {tilde over(H)} becomes:{tilde over (H)}=H _(N) _(RX) _(×8) Q _(8×M)  (2){tilde over (H)} is an N_(RX)×M matrix. Using SVD, it can be written as:{tilde over (H)}=Ũ _(N) _(RX) _(×N) _(RX) Σ_(N) _(RX) _(×M) V _(M×M)⁺  (3)

The receiver performs an SVD of channel {tilde over (H)} and returns theequivalent of the matrices {tilde over (Σ)}_(N) _(RX) _(×M) and {tildeover (V)}_(M×M). At the same time, the channel {tilde over (H)} can alsobe written as:

$\begin{matrix}\begin{matrix}{\overset{\sim}{H} = {H_{N_{RX} \times 8}Q}} \\{= {U\;\Sigma\; V_{8}^{+}Q}}\end{matrix} & (4)\end{matrix}$Next, we observe that the M×M matrix {tilde over (H)}⁺{tilde over (H)}can be written in two equivalent ways.First, from (3), {tilde over (H)}⁺{tilde over (H)} is equal to:{tilde over (H)} ⁺ {tilde over (H)}={tilde over (V)} _(M×M){tilde over(Σ)}_(M×N) _(RX) ⁺{tilde over (Σ)}_(N) _(RX) _(×M) {tilde over (V)}_(M×M) ⁺  (5)On the other hand, from (4), {tilde over (H)}⁺{tilde over (H)} is alsoequal to:

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{H}}^{+}\overset{\sim}{H}} = {Q^{+}V_{8}\Sigma^{+}\Sigma\; V_{8}^{+}Q}} \\{= {Q^{+}{PQ}}}\end{matrix} & (6)\end{matrix}$

Expression (5) is fully determined by the sounding feedback provided bythe receiver, since it only depends on {tilde over (V)} and {tilde over(Σ)}. Expression (6) is a linear combination of the elements of V₈Σ⁺ΣV₈⁺. The matrix P=V₈Σ⁺ΣV₈ ⁺ is an 8×8 matrix with a singular valuedecomposition that has a V and Σ matrix that are identical to the V andΣ matrix of the original channel H_(N) _(RX) _(×8) (see (1)). If we cansolve the equality between (5) and (6) for the components of P, we haveaccess to the eight-stream information we are trying to obtain. Thesolution for V is not unique. If V is a V-matrix for P, then the matrixVD will also be a V-matrix as long as D is a diagonal unitary matrix.This undetermined phase is not an issue for beamforming or MU-MIMOhowever. Unfortunately, the equality of (5) and (6) is under-determinedand does not allow us to derive the full matrix P. This can be resolvedby doing four independent sounding and combining the results asdiscussed below.

By varying the spatial mapping matrix Q, we get access to differentparts of the matrix P. By choosing an appropriate set of fourindependent mapping matrices, we will ultimately have enough equationsto recover the full matrix P. The following set of four mapping matriceswill serve this purpose:

$\begin{matrix}\begin{matrix}{{Q_{1} = \begin{bmatrix}\delta_{M \times M} \\0_{M \times M}\end{bmatrix}}\mspace{315mu}} & (a) \\{{Q_{2} = \begin{bmatrix}0_{M \times M} \\\delta_{M \times M}\end{bmatrix}}\mspace{315mu}} & (b) \\{{Q_{3} = {\frac{1}{\sqrt{2}}\begin{bmatrix}\delta_{M \times M} \\\delta_{M \times M}\end{bmatrix}}}\mspace{315mu}} & (c) \\{{Q_{4} = {\frac{1}{\sqrt{2}}\begin{bmatrix}\delta_{M \times M} \\{j\;\delta_{M \times M}}\end{bmatrix}}}\mspace{315mu}} & (d)\end{matrix} & (7)\end{matrix}$Note that it may be possible to choose a different set of four mappingmatrices, but we will use the set shown in (7) going forward. FIGS.3A-3D show the stream-to-antenna mapping associated with each of thefour linearly independent composite soundings and corresponding one ofthe four mapping matrices (7).

It is convenient to write the matrix P as a block-diagonal matrix:

$\begin{matrix}{P = \begin{bmatrix}P^{(11)} & P^{(12)} \\P^{(21)} & P^{(22)}\end{bmatrix}} & (8)\end{matrix}$Each of the blocks in (8) is a 4×4 matrix. With this notation, we get(using (6) for each of the mapping matrices):

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{H}}_{1}^{+}{\overset{\sim}{H}}_{1}} = {Q_{1}^{+}{PQ}_{1}}} \\{= P_{{1:4},{1:4}}} \\{= P^{(11)}}\end{matrix} & (9)\end{matrix}$

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{H}}_{2}^{+}{\overset{\sim}{H}}_{2}} = {Q_{2}^{+}{PQ}_{2}}} \\{= P_{{5:8},{5:8}}} \\{= P^{(22)}}\end{matrix} & (10)\end{matrix}$

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{H}}_{3}^{+}{\overset{\sim}{H}}_{3}} = {Q_{3}^{+}{PQ}_{3}}} \\{= \frac{P^{(11)} + P^{(22)} + P^{(12)} + P^{(21)}}{2}}\end{matrix} & (11)\end{matrix}$

$\begin{matrix}\begin{matrix}{{{\overset{\sim}{H}}_{4}^{+}{\overset{\sim}{H}}_{4}} = {Q_{4}^{+}{PQ}_{4}}} \\{= \frac{P^{(11)} + P^{(22)} + {j\left( {P^{(12)} - P^{(21)}} \right)}}{2}}\end{matrix} & (12)\end{matrix}$

Each of the matrices {tilde over (H)}₁ ⁺{tilde over (H)}₁, {tilde over(H)}₂ ⁺{tilde over (H)}₂, {tilde over (H)}₃ ⁺{tilde over (H)}₃ and{tilde over (H)}₄ ⁺{tilde over (H)}₄ is fully known from the four-streamsoundings feedback. From (9)-(12), we can solve for the block componentsof P (see (8)):

$\begin{matrix}{{P^{(11)} = {{\overset{\sim}{H}}_{1}^{+}{\overset{\sim}{H}}_{1}}}{P^{(22)} = {{\overset{\sim}{H}}_{2}^{+}{\overset{\sim}{H}}_{2}}}{P^{(12)} = {{{\overset{\sim}{H}}_{3}^{+}{\overset{\sim}{H}}_{3}} + {j{\overset{\sim}{H}}_{4}^{+}{\overset{\sim}{H}}_{4}} - {\frac{\left( {1 - j} \right)}{2}\left( {{{\overset{\sim}{H}}_{1}^{+}{\overset{\sim}{H}}_{1}} + {{\overset{\sim}{H}}_{2}^{+}{\overset{\sim}{H}}_{2}}} \right)}}}{P^{(21)} = {{{{\overset{\sim}{H}}_{3}^{+}{\overset{\sim}{H}}_{3}} - {j{\overset{\sim}{H}}_{4}^{+}{\overset{\sim}{H}}_{4}} - {\frac{\left( {1 - j} \right)}{2}\left( {{{\overset{\sim}{H}}_{1}^{+}{\overset{\sim}{H}}_{1}} + {{\overset{\sim}{H}}_{2}^{+}{\overset{\sim}{H}}_{2}}} \right)}} = P^{{(12)}^{+}}}}} & (13)\end{matrix}$From (13), we can reconstruct the entire matrix P. Once P is known, wehave access to the V and Σ matrix of the original channel H_(N) _(RX)_(×8). This requires an SVD on the 8×8 matrix P. Note however that P ishermitian and that we only expect N_(RX) non-zero singular modes. Takinginto account this information can reduce the complexity of the SVD thatis involved in the calculation (see example below).

Example: N_(RX)=1

The task of performing an SVD on the 8×8 matrix P looks very complicatedat first. However, we can exploit a priori knowledge about the rank ofthe matrix. Take the example of a receiver with a single antenna(N_(RX)=1). In that case, the 8×8 matrix has rank 1 and can therefore bewritten in the following form:P=Vσ ² V ⁺  (14)In (14), V is a normalized 8×1 dimensional vector and σ is a singlepositive real number. Instead of having to perform a full 8×8 SVD, weonly have to solve for σ and V.

The value of σ can be found by observing that:Trace(P)=σ²  (15)Since P has rank 1, all columns are either linearly dependent or zero.So, any non-zero column of P will form a complete basis for the non-nullspace of P. A solution for V is then given by:

$\begin{matrix}{V = \frac{P\left( {:{,1}} \right)}{\sigma\sqrt{P\left( {1,1} \right)}}} & (16)\end{matrix}$

Provided that P(1,1) is different from zero. If not, another column canbe chosen to find the value of V. In this case, one can even forgo thecomplete calculation of P (as in (8) and (13)), since only a singlecolumn is needed to find the desired decomposition (14). Even so, weneed to perform the four different soundings since it is not possible toget a complete column without them. Likewise, for other values ofN_(RX), suitable simplifications should avoid the need to perform a full8×8 SVD on P

Receiver Cooperation

While the technique discussed above is mainly intended to be transparenttowards existing implementations, one could conceive a version of thetechnique that relies on cooperation from the receiver. In thisalternate embodiment of the invention, if the receiver is aware of themethod being used, the different mapping matrices could be combined in asingle sounding. Specifically, instead of sounding four streams in asounding packet, the sounding packet could contain e.g. eight trainingsymbols. The first four training symbols are mapped with one of thematrices in (7) and the next four with a different matrix. The receiverthen processes the eight training symbols as two groups of four, sendingback two sets of four-stream sounding feedback. The originator can thencombine the sounding feedback as described above. The advantages arethat less time is taken up doing the sounding and receiving thefeedback. For the receiver, the advantage is that its SVD implementationcould potentially be simpler, while still allowing the transmitter toextract the necessary information.

The techniques describe above reconstructs sounding feedback equivalentto the feedback that would be generated by an eight-stream feedbackcapable device. In some cases, complete information may not be needed.Maybe enough relevant information can be extracted from a singlesounding with less than eight streams to get some performanceimprovement at lower cost. The method then extends to any case wherepartial sounding can be used to derive some information that is relevantfor an eight-stream system. The above discussed method allows aneight-antenna system to extract the equivalent of eight-stream feedbackfrom receivers that are only capable of providing four-stream feedback.The key to making this work is to perform successive soundings withsuitable chosen spatial mapping matrices and to combine the results asdescribed.

Sequential Sounding Mapping Matrices

Equations (7a) to (7d) give a possible set of spatial mapping matricesthat can be used to reconstruct 8-stream feedback from multiple 4-streamsoundings. This set is not the only possible set of mapping matricesthat can be used. Below, we'll derive a more general criterion todetermine when a set of mapping matrices can be used in sequentialsounding. The goal of sequential/composite sounding is to reconstruct an8×8 matrix P. To find the solution, a set of four 4×4 matrices is given.M _(α) =Q _(α) ⁺ PQ _(α),=␣1, . . . ,4  (17)The matrices Q_(α) are known and are the different spatial mappingmatrices used in the sequential sounding. The matrices M_(α) areconstructed from the sounding feedback and are known as well. Not everyset of four spatial mapping matrices Q_(i) will allow us to fullyreconstruct the matrix P. Below we'll discuss the criteria for a goodset of matrices.

Equation (17) is essentially a set of linear equations in the elementsof P (although it is quadratic in Q_(α)). We can rewrite this as:

$\begin{matrix}\begin{matrix}{\left( M_{\alpha} \right)_{ij} = {\sum\limits_{k,{l = 1}}^{8}{\left( Q_{\alpha}^{*} \right)_{ki}{P_{kl}\left( Q_{\alpha} \right)}_{lj}}}} \\{= {\sum\limits_{k,{l = 1}}^{8}{\left( Q_{\alpha}^{*} \right)_{ki}\left( Q_{\alpha} \right)_{lj}P_{kl}}}} \\{= {\sum\limits_{({k,l})}{c_{{({i,j})}{({k,l})}}^{\alpha}P_{({k.l})}}}}\end{matrix} & (18)\end{matrix}$We can now think of each (M_(α))_(ij) as a 16×1 vector, c_((i,j)(k,l))^(α) as a 16×64 matrix and P_((k,l)) as a 64×1 vector. The fourequations in (17) are therefore equivalent to:M _(64×1) =C _(64×64) P _(64×1)  (19)The 64×1 vector M_(64×1) can be uniquely constructed from the originalfeedback matrices M_(α). Likewise, the 64×64 matrix C_(64×64) has aunique one-to-one relation with the chosen spatial mapping matrices. So,for a given set of spatial mapping matrices, both M_(64×1) and C_(64×64)are known.

In the form (19), the original problem has been reformulated as a set of64 equations in 64 unknowns (P_(64×1)). Whether a solution exists (andtherefore, whether sequential sounding will work for the given set ofspatial mapping matrices) depends on the properties of C_(64×64). IfC_(64×64) is non-singular, the solution to (19) is given by:P _(64×1)=(C _(64×64))⁻¹ M _(64×1)  (20)Once P_(64×1) is known, the desired matrix P can be directly derivedfrom it.

To determine whether a set of matrices Q_(α) constitutes a good set ofmapping matrices for sequential sounding, one can therefore perform thefollowing test:

-   -   Construct the matrix C_(64×64) corresponding to the matrices        Q_(α).    -   Verify that the matrix C_(64×64) is non-singular (and        well-conditioned)        If C_(64×64) passes the test, the set of matrices Q_(α) is an        acceptable choice for the spatial mapping matrices in a        sequential sounding. Note that this does not provide a procedure        for constructing the matrices Q_(α), but it does suggest that a        large set of matrices can be found that would work for        sequential sounding. Ideally, the matrices Q_(α) should be        chosen such that the inversion problem (20) can be solved as        easily as possible. The matrices Q_(α) that are shown in (7a) to        (7d) have the advantage that they greatly simplify the inversion        in (20).

Number of Soundings for Stations with More than One Antenna

Expressing the problem as a set of linear equations also allows us tofind the number of soundings that is needed in a general case. FromEquation (19), we see that we need to solve for up to 64 unknowns. Eachsounding produces a 4×4 matrix M_(α) (see (17)) which ends upcontributing elements to the 64×1 vector M_(64×1). Since each of thematrices M_(α) has 16 elements, we need four soundings giving us fourM_(α) to have all elements of M_(64×1). In a general case, let's assumethat the transmit side (AP) is capable of sounding and using a maximumnumber of streams equal to N_(STS,TX), which may be equal to the numberof transmit antennas that the device has. The client side can provide amaximum of N_(STS,RX) streams. Expressed as a set of linear equations,we would need N_(STS,TX) ² equations to solve for the N_(STS,TX) ²elements of the matrix P. On the other hand, each sounding provides uswith N_(STS,RX) ² elements of the “vector” M. Therefore, the minimumnumber of soundings that is needed to make it possible to solve theequation (3) is given by:

$\begin{matrix}\left\lceil \frac{N_{{STS},{TX}}^{2}}{N_{{STS},{RX}}^{2}} \right\rceil & (21)\end{matrix}$For each sounding, we also need a Spatial mapping matrix Q_(α).

Number of Soundings for Single-Antenna Clients

Sequential sounding for 1-antenna clients that provide N_(STS,RX) streamfeedback provides an opportunity for reduction in the number ofsequential/composite soundings. In this case, we don't have to build upthe matrix P as an intermediate to finding the solution. Instead, wedirectly build up the channel H. The minimum number of steps that isneeded for this case is simply:

$\begin{matrix}\left\lceil \frac{N_{{STS},{TX}}}{N_{{STS},{RX}}} \right\rceil & (22)\end{matrix}$

Additional Soundings:

For certain implementations of the composite sounding method describedherein, such as the simplified method for single antenna clientsdescribed below, there could be an unknown phase offset betweendifferent partial sounding feedbacks. These phase offsets have to bedetermined and removed before expansion into the full beamformingmatrix. In those cases, we can perform additional partial soundings suchthat there is an overlap in information about the streams that arecontained in different partial feedbacks. For instance, one partialsounding feedback could contain information about streams one to fourand the second partial sounding feedback could contain information aboutstreams three to six. The duplication of the feedback on streams threeand four allows the determination of the unknown phase between the firstand second partial sounding feedback.

Composite Sounding for Single Antenna Client Stations

We previously described a method that allows the AP to reconstructeight-stream feedback from four different sets of four-stream feedbackresponses. In practice, this means the client STAs need to be soundedfour times. It is worth noting that for one-antenna clients, the samemultiple-sounding approach can be simplified to two soundings. Thisresult is specific for single-antenna stations only and we'll discuss itin more detail below. Many of the target client devices for MU-MIMO willbe single-antenna devices. We can assume that many of these devices willbe capable of providing four-stream (MU-type) feedback. It's also fairto assume that in the near future, they will not be capable of providingeight-stream feedback. The channel between the (eight-antenna) AP andthe client is a 1×8 matrix. Conceptually, the AP needs to know the E andV-matrix of the SVD of the 1×8 matrix. For a 1×8 matrix, the 1×1U-matrix of the SVD is trivial and can be assumed to be 1. This meansthat the channel can be written as:H _(1×8)=Σ_(1×8) V _(8×8) ⁺  (23)Since Σ_(1×8) has only one non-zero component, only the first row ofV_(8×8) ⁺ actually matters. We can rewrite (23) as:(H _(1×8))⁺=σ₁ V _(8×1)  (24)In other words: the V-matrix of the 1×8 channel is simply a scaledversion of the conjugate of H_(1×8). The scaling can be found by takinginto account that:(V _(8×1))⁺ V _(8×1)=1  (25)Therefore:H _(1×8)(H _(1×8))⁺=(V _(8×1))⁺σ₁ ² V _(8×1)=σ₁ ²  (26)Or:σ₁ =|H _(1×8)|  (27)

So, for a 1×8 channel, the full SVD can be found by the following steps:

-   -   1. Assume that the U-matrix is 1    -   2. Calculate σ₁ (and therefore the full Σ-matrix) as in (27)    -   3. Calculate the V-matrix by dividing H⁺ by σ₁

Conversely, given only the Σ-matrix and the V-matrix, we can reconstructthe full channel by multiplying σ₁ and V⁺. This is only possible for1×N_(TX) channels because only for this case, we do not requireknowledge of the U-matrix.

Taking this into account, sequential sounding can be used as follows.First sound with the following 8×4 spatial mapper matrix, which mapsfour spatial streams to the eight antennas:

$\begin{matrix}{Q^{(1)} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}} & (28)\end{matrix}$The channel observed by the single-antenna client is H_(1×8)Q⁽¹⁾.Note that:H _(1×8) Q ⁽¹⁾ =[h ₁ h ₂ h ₃ h ₄]  (29)The client device provides sounding feedback consisting of the scalarvalue σ₁ ⁽¹⁾ and the 4×1 matrix V⁽¹⁾ such that:[h ₁ h ₂ h ₃ h ₄]=σ₁ ⁽¹⁾(V ⁽¹⁾)⁺  (30)For the second sounding, we use the spatial mapping matrix shown below:

$\begin{matrix}{Q^{(2)} = \begin{bmatrix}0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}} & (31)\end{matrix}$The channel observed by the single-antenna client is now H_(1×8)Q⁽²⁾.Note that:H _(1×8) Q ⁽²⁾ =[h ₅ h ₆ h ₇ h ₈]  (32)

The client device provides sounding feedback consisting of the scalarvalue σ₁ ⁽²⁾ and the 4×1 matrix V⁽²⁾ such that:[h ₅ h ₆ h ₇ h ₈]=σ₁ ⁽²⁾(V ⁽²⁾)⁺  (33)From (30) and (33), we now know the complete 1×8 channel H_(1×8):H _(1×8)=[σ₁ ⁽¹⁾(V ⁽¹⁾)⁺σ₁ ⁽²⁾(V ₁ ⁽²⁾)⁺]  (34)The SVD can be found as described earlier for 1×N_(TX) matrices. Notethat the mapping matrices (28) and (31) are not the only matrices thatcan be used. Pretty much any two matrices that together form 8 linearlyindependent vectors can be used to reconstruct H_(1×8). Obtaining aneight-stream feedback from four-stream devices by combining feedbackfrom a number of independent four-stream soundings has been discussedabove. In its full form, this required four four-stream soundings to getthe equivalent of one eight-stream sounding. We show here that themethod can be simplified for single-antenna devices, such that only twoinstead of four soundings are needed.

Partial Channel Matrices Sounding Feedback:

The second approach to reconstructing a fully dimensioned beamformingmatrix for the WAP relies on the availability of “H” matrices. Thisapproach is compatible with a station which provide “raw” measuredchannel information. This means the channel measurement is not processedor transformed before it is fed back to the WAP that originated thesounding. Consider an example where the sounding initiator has eighttransmit antennas, but the responder can only feedback information forat most four antennas in a single sounding. The channel can be denotedas:H(1:N _(RX),1:8)  (35)where N_(RX) is the number of receive antennas of the responder. In thiscase, the sounding initiator could first let the responder estimate thechannel from its first four antennas. This would lead to the feedback ofH(1:N_(RX), 1:4). In a second sounding, the sounding initiator can thenlet the responder estimate the channel from its second set of fourantennas. This leads to the feedback of H(1:N_(RX), 5:8). After bothfeedbacks have been received, the sounding initiator can reconstruct thecomplete channel by simply concatenating the two feedbacks, i.e.:

$\begin{matrix}{{H\left( {{1:N_{RX}},{1:8}} \right)} = \begin{bmatrix}{H\left( {{1:N_{RX}},{1:4}} \right)} \\{H\left( {{1:N_{RX}},{5:8}} \right)}\end{bmatrix}} & (36)\end{matrix}$

Normalizing Sequential Soundings:

An 8-antenna AP needs 8-stream feedback to fully characterize thechannel between the AP and the client. Having this 8-dimensionalfeedback available enables the most efficient use of beamforming andMU-MIMO precoding. In practice, many IEEE 802.11ac clients are limitedin their ability to process sounding packets. Today, most clients willonly provide feedback to sounding packets with a maximum of fourstreams. As discussed above, it is possible to obtain higher-dimensionalsounding feedback by combining feedback from several lower-dimensionalfeedbacks. For instance, by sounding a client with four successivefour-streams sounding packets using different spatial mapping matrices,it is possible to build up the equivalent 8-stream sounding feedback.This is called composite sequential sounding. However, this scheme hasone important problem in practice, specifically the changes in either orboth amplitude and phase of one sounding in a sequence relative toanother. Amplitude differences between sequential soundings may arisefrom differences in automatic gain control (AGC) in the receiver of thestation being sounded. Phase differences between sequential soundingsmay arise from differences in symbol window boundary determination madein the baseband of the receiver of the station being sounded. Eitheramplitude or phase induced differences between the sequential soundings,will result in significant performance degradation in compositesequential sounding scheme.

In an embodiment of the invention, the use of one or more referencetones with fixed indices and at least one fixed reference SMM acrosssoundings in a sequence is used to normalize any differences in eitheror both amplitude and phase of the sequential sounding feedback beforecombining them into a composite sounding.

Sequential Sounding and AGC Gain Compensation:

In practice, the variations in the AGC of the receiver processingsequential soundings can create problems. Typical receiver has an AGCwith a fixed gain throughout a packet. However, the gain might changebetween different sounding packets. In this case, instead of gettingfeedback on the virtual channel, {tilde over (H)}, we get feedback onthe virtual channel multiplied by AGC gain a.Ĥ=aĤ  (37)This results in the following set of modified equations for P matrix ascompared to the P matrix discussed above:

$\begin{matrix}\begin{matrix}{\frac{{\hat{H}}_{1}^{+}{\hat{H}}_{1}}{a_{1}^{2}} = {Q_{1}^{+}{PQ}_{1}}} \\{= P_{{1:4},{1:4}}} \\{= P^{(11)}}\end{matrix} & (38)\end{matrix}$

$\begin{matrix}\begin{matrix}{\frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{a_{2}^{2}} = {Q_{2}^{+}{PQ}_{2}}} \\{= P_{{5:8},{5:8}}} \\{= P^{(22)}}\end{matrix} & (39)\end{matrix}$

$\begin{matrix}\begin{matrix}{\frac{{\hat{H}}_{3}^{+}{\hat{H}}_{3}}{a_{3}^{2}} = {Q_{3}^{+}{PQ}_{3}}} \\{= \frac{P^{(11)} + P^{(22)} + P^{(12)} + P^{(21)}}{2}}\end{matrix} & (40)\end{matrix}$

$\begin{matrix}\begin{matrix}{\frac{{\hat{H}}_{4}^{+}{\hat{H}}_{4}}{a_{4}^{2}} = {Q_{4}^{+}{PQ}_{4}}} \\{= \frac{P^{(11)} + P^{(22)} + {j\left( {P^{(21)} + P^{(12)}} \right)}}{2}}\end{matrix} & (41)\end{matrix}$From (38-41), we can solve for the block components of P:

$\begin{matrix}{{P^{(11)} = \frac{{\hat{H}}_{1}^{+}{\hat{H}}_{1}}{a_{1}^{2}}}{P^{(22)} = \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{a_{2}^{2}}}} & (42)\end{matrix}$

$\begin{matrix}{\mspace{85mu}{{P^{(12)} = {\frac{{\hat{H}}_{3}^{+}{\hat{H}}_{3}}{a_{3}^{2}} + {j\frac{{\hat{H}}_{4}^{+}{\hat{H}}_{4}}{a_{4}^{2}}} - {\frac{\left( {1 + j} \right)}{2}\left( {\frac{{\hat{H}}_{1}^{+}{\hat{H}}_{1}}{a_{1}^{2}} + \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{a_{2}^{2}}} \right)}}}{P^{(21)} = {{\frac{{\hat{H}}_{3}^{+}{\hat{H}}_{3}}{a_{3}^{2}} - {j\frac{{\hat{H}}_{4}^{+}{\hat{H}}_{4}}{a_{4}^{2}}} - {\frac{\left( {1 + j} \right)}{2}\left( {\frac{{\hat{H}}_{1}^{+}{\hat{H}}_{1}}{a_{1}^{2}} + \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{a_{2}^{2}}} \right)}} = P^{{(12)}^{+}}}}}} & (43)\end{matrix}$

If we just scale the P matrix with the gain square of the firstsounding, we geta ₁ ² P ⁽¹¹⁾ =Ĥ ₁ ⁺ H ₁  (44)

$\begin{matrix}{{a_{1}^{2}P^{(22)}} = \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{\left( \frac{a_{2}}{a_{1}} \right)^{2}}} & (45)\end{matrix}$

$\begin{matrix}{\mspace{79mu}{{{a_{1}^{2}P^{(12)}} = {\frac{{\hat{H}}_{3}^{+}{\hat{H}}_{3}}{\left( \frac{a_{3}}{a_{1}} \right)^{2}} + {j\frac{{\hat{H}}_{4}^{+}{\hat{H}}_{4}}{\left( \frac{a_{4}}{a_{1}} \right)^{2}}} - {\frac{\left( {1 + j} \right)}{2}\left( {{{\hat{H}}_{1}^{+}{\hat{H}}_{1}} + \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{\left( \frac{a_{2}}{a_{1}} \right)^{2}}} \right)}}}{{a_{1}^{2}P^{(21)}} = {{\frac{{\hat{H}}_{3}^{+}{\hat{H}}_{3}}{\left( \frac{a_{3}}{a_{1}} \right)^{2}} - {j\frac{{\hat{H}}_{4}^{+}{\hat{H}}_{4}}{\left( \frac{a_{4}}{a_{1}} \right)^{2}}} - {\frac{\left( {1 + j} \right)}{2}\left( {{{\hat{H}}_{1}^{+}{\hat{H}}_{1}} + \frac{{\hat{H}}_{2}^{+}{\hat{H}}_{2}}{\left( \frac{a_{2}}{a_{1}} \right)^{2}}} \right)}} = {a_{1}^{2}P^{{(12)}^{+}}}}}}} & (46)\end{matrix}$

To calculate the P matrix which is scaled by the AGC gain of the firstsounding, the transmitter needs to know the ratios (a₂/a₁), (a₃/a₂), and(a₄/a₃). To solve this problem, we reserve few subcarriers, a.k.a.tones, of the channel being sounded as reference subcarriers for AGCgain compensation. Let's call these reference tones, with fixed indicesk_(r). In an embodiment of the invention, for those reference tones, weuse only one fixed spatial mapping matrix Q₁, a.k.a. “Q_(ref)”, for allreference tones and all of the four consecutive soundings. If we assumeall the consecutive four soundings happened within channel coherencetime, then the following results are obtained for reference sequences.

$\begin{matrix}\begin{matrix}{\frac{{{\hat{H}}_{1}^{+}\left( k_{r} \right)}{{\hat{H}}_{1}\left( k_{r} \right)}}{a_{1}^{2}} = {Q_{1}^{+}{P\left( k_{r} \right)}Q_{1}}} \\{= \frac{{{\hat{H}}_{1}^{+}\left( k_{r} \right)}{{\hat{H}}_{1}\left( k_{r} \right)}}{a_{2}^{2}}} \\{= \frac{{{\hat{H}}_{1}^{+}\left( k_{r} \right)}{{\hat{H}}_{1}\left( k_{r} \right)}}{a_{3}^{2}}} \\{= \frac{{{\hat{H}}_{1}^{+}\left( k_{r} \right)}{{\hat{H}}_{1}\left( k_{r} \right)}}{a_{4}^{2}}}\end{matrix} & (47)\end{matrix}$

Since the multiplication by a constant only affects the diagonal matrixin SVD, we can re-write these equations above as follows

$\begin{matrix}\begin{matrix}{\frac{{{\hat{\Sigma}}_{1}^{+}\left( k_{r} \right)}{{\hat{\Sigma}}_{1}\left( k_{r} \right)}}{a_{1}^{2}} = {Q_{1}^{+}{P\left( k_{r} \right)}Q_{1}}} \\{= \frac{{{\hat{\Sigma}}_{1}^{+}\left( k_{r} \right)}{{\hat{\Sigma}}_{1}\left( k_{r} \right)}}{a_{2}^{2}}} \\{= \frac{{{\hat{\Sigma}}_{1}^{+}\left( k_{r} \right)}{{\hat{\Sigma}}_{1}\left( k_{r} \right)}}{a_{3}^{2}}} \\{= \frac{{{\hat{\Sigma}}_{1}^{+}\left( k_{r} \right)}{{\hat{\Sigma}}_{1}\left( k_{r} \right)}}{a_{4}^{2}}}\end{matrix} & (48)\end{matrix}$

Each of the diagonal matrices has N_(RX) singular values. Therefore, wecan calculate the ratio of AGC gains between successive soundings asfollows:

$\begin{matrix}{\frac{a_{2}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)} = {\frac{1}{N_{RX}}{\sum\limits_{i = 1}^{N_{RX}}\frac{{\sigma\left( k_{r} \right)}_{i}^{(2)}}{{\sigma\left( k_{r} \right)}_{i}^{(1)}}}}} & (49)\end{matrix}$

$\begin{matrix}{\frac{a_{3}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)} = {\frac{1}{N_{RX}}{\sum\limits_{i = 1}^{N_{RX}}\frac{{\sigma\left( k_{r} \right)}_{i}^{(3)}}{{\sigma\left( k_{r} \right)}_{i}^{(1)}}}}} & (50)\end{matrix}$

$\begin{matrix}{\frac{a_{4}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)} = {\frac{1}{N_{RX}}{\sum\limits_{i = 1}^{N_{RX}}\frac{{\sigma\left( k_{r} \right)}_{i}^{(4)}}{{\sigma\left( k_{r} \right)}_{i}^{(1)}}}}} & (51)\end{matrix}$where σ(k_(r))_(i) ^((n)) means i^(th) singular value of the n^(th)sounding feedback at reference tone or subcarrier k_(r). Finally, theAGC compensation corresponds to scaling the matrices of each sounding asfollows before constructing the P matrix

$\begin{matrix}\begin{matrix}{\hat{\Sigma}}_{1} & (a) \\{\frac{a_{1}\left( k_{r} \right)}{a_{2}\left( k_{r} \right)}{\hat{\Sigma}}_{2}} & (b) \\{\frac{a_{1}\left( k_{r} \right)}{a_{3}\left( k_{r} \right)}{\hat{\Sigma}}_{3}} & (c) \\{{\frac{a_{1}\left( k_{r} \right)}{a_{4}\left( k_{r} \right)}{\hat{\Sigma}}_{4}}\mspace{295mu}} & (d)\end{matrix} & (52)\end{matrix}$To get a more reliable gain compensation, this can be done over multiplereference tones interlaced with the remaining tones in the communicationchannel being sounded. In that case, the gain ratios can be estimated as

$\begin{matrix}\begin{matrix}{\frac{a_{2}}{a_{1}} = {\frac{1}{K_{r}}{\sum\limits_{i = 1}^{K_{r}}\frac{a_{2}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)}}}} & (b) \\{\frac{a_{3}}{a_{1}} = {\frac{1}{K_{r}}{\sum\limits_{i = 1}^{K_{r}}\frac{a_{3}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)}}}} & (c) \\{{\frac{a_{4}}{a_{1}} = {\frac{1}{K_{r}}{\sum\limits_{i = 1}^{K_{r}}\frac{a_{4}\left( k_{r} \right)}{a_{1}\left( k_{r} \right)}}}}\mspace{185mu}} & (d)\end{matrix} & (53)\end{matrix}$

Then the AGC gain normalization applied to the sounding feedback to thefirst, second, third and fourth set of per tone SNR matrices of an802.11ac compliant explicit sounding feedback becomes:(1),(a ₁ /a ₂),(a ₁ /a ₃),(a ₁ /a ₄)  (54)After such normalization the composite soundings are expanded into afull beamforming matrix as discussed above.

Sequential Sounding and Phase Compensation:

Where the sequential sounding feedback is in the form of sequentialpartial channel matrices, i.e. “H” matrices, as in an IEEE 802.11ncompliant sounding, it is possible that an unknown phase differenceexists between the sequential sounding feedbacks. In that case, thisphase difference must be resolved before the partial feedbacks can beconcatenated as discussed in 35-36 above. Resolving the phase differencecan be accomplished by using one or more reference tones with fixed SMMin each of the sequential soundings. Variations in the phase of theconstellation points of the “H” matrices in the sounding feedback foreach reference tone in the sequence of soundings can be used tonormalize the phase differences between successive soundings beforeconcatenation across all remaining tones in each sounding relative tothe other sounding(s) in the set of soundings. The difference in phaseon the reference tone across the successive soundings gives anindication of the phase difference between the different soundings whichare in turn applicable to all remaining tones in each set of tones ineach sounding.

FIGS. 4A-B are system views of a multi-user (MU) multiple-inputmultiple-output (MIMO) downlink to stations with constrained soundingcapabilities using a prior art sounding (FIG. 4A) and a composite set ofsoundings in accordance with an embodiment of the invention (FIG. 4B)respectively.

FIG. 4A shows the Prior Art WAP 100 discussed above in connection withFIGS. 1A-B effecting an MU-MIMO downlink to both stations 420 and 440.Station 420 is identified as a 2×2 device having two antenna andsupporting up to 2 streams of downlink and uplink communications. Thestation has a baseband section 422 and a radio frequency (RF) section430 coupled to 2 multiple-input multiple-output (MIMO) antennas 432. Thebaseband receive portion is shown and the corresponding transmit portionis not. The discrete Fourier Transform (DFT) modules 428A-B are coupledto a corresponding one of the two antenna 432, to transform receivedcommunications from the time to the frequency domain. The output bins426A-B of the DFT modules are coupled to the input of the equalizer 424.The sounding feedback capabilities of the station 420 do not match theWAPs capabilities in terms of number of streams and antennas. The WAPcould handle up to 4 streams on a downlink sounding, but can't do sobecause the station is only capable of providing sounding feedback forup to two streams. Station 440 is also a 2×2 device. The station has abaseband section 442 and a radio frequency (RF) section 445 coupled to 2multiple-input multiple-output (MIMO) antennas 452. The DFT modules448A-B are coupled to a corresponding one of the two antenna 452, totransform received communications from the time to the frequency domain.The output bins 446A-B of the DFT modules are coupled to the input ofthe equalizer 444. The sounding feedback capabilities of the station 440do not match the WAPs capabilities in terms of number of streams andantennas. The WAP could handle up to 4 streams on a downlink sounding,but can't do so because the station is only capable of providingsounding feedback for up to two streams. After the single sounding ofeach station by the WAP a 1×1 link from the WAP to each station isestablished using the under dimensioned sounding responses 400 from eachstation. The beamforming strength 457 of the MU-MIMO downlink is limitedby the under dimensioned sounding available from both stations.

FIG. 4B shows the WAP 200 shown in FIG. 2A-2B establishing a MU-MIMOdownlink to the above discussed stations 420 and 440. A compositesounding is used to obtain the linearly independent set of four partialsounding feedback matrices 434A-D and 454A-D from stations 420 and 440respectively. These in turn are expanded into corresponding fullbeamforming matrices 4326 and 456 for station 420 and 440 respectively.These are then used to spatially map orthogonal communications ofstreams “ab” to station 420 and “cd” to station 440. The channels oneach link between the WAP and a corresponding one of the stations arefully characterized and thus each link is able to support 2×2communications. The beamforming strength and focus 458 of the MU-MIMOdownlink is enhanced by virtue of the fully dimensioned beamformingmatrices 436, 456 determined by the WAP. The WAP 200 expands thecomposite set of four partial sounding feedback matrices 434A-D fromstation 420 to obtain full beamforming matrix 436. The WAP 200 expandsthe composite set of four partial sounding feedback matrices 454A-D fromstation 440 to obtain full beamforming matrix 456.

In an embodiment of the invention where one of the MU-MIMO downlinktarget stations has sounding feedback capability in terms of the numberof characterizable streams, which matches that of the WAP only a singlesounding may be required, with the composite sounding devoted to fullycharacterizing the downlink channel to the remaining station withlimited sounding feedback capability.

FIGS. 5A-C are signal diagrams of a MIMO downlink to a station withconstrained sounding capabilities: without beamforming (FIG. 5A), withbeamforming resulting from a single prior art sounding (FIG. 5B) andwith beamforming resulting from a composite sounding in accordance withan embodiment of the invention (FIG. 5C).

FIG. 5A is a signal diagram of a MIMO downlink without beamforming to astation with constrained sounding capabilities. Four WAP antennas 512A-Dare shown initiating transmission (TX) of a common sinusoidal waveformof equal amplitude and frequency over each of the antennas starting atthe same time, tabcd. Each signal exhibits a different delay andattenuation upon arrival at the RX antenna and summer. This is due tothe differences in the individual paths or channel between each of thefour TX antenna and the single receive (RX) antenna and adder 520. Theresult is that there is considerable destructive interference betweeneach upon arrival at the RX antenna and adder. The aggregate signal 522at the output of the adder is severely attenuated in amplitude as shown,and may also exhibit overtones or higher frequency harmonics than theoriginal signal due to different transit times and associated phaseshifts relative to the other signals.

FIG. 5B is a signal diagram of a MIMO downlink with beamforming to astation with constrained sounding capabilities. The beamforming 530 ispartial since the channel sounding is under dimensioned due to theinability of the receiver to fully characterize all four streamstransmitted by the WAP. In the example shown, the associated soundingfeedback is assumed to be two streams. Four WAP antennas 512A-D areshown initiating transmission (TX) of the common sinusoidal waveform ofequal amplitude and frequency at two discrete times, tab and tcd asdetermined from the partial sounding. The staggered transmission timesare designed to align the received signals with one another so that theyconstructively interfere with one another. However, as shown in theresultant sum signal 532 at the output of adder 520 the underdimensioned sounding limits the effectiveness of the beamforming.

FIG. 5C is a signal diagram of a MIMO downlink without beamforming to astation with constrained sounding capabilities. Four WAP antennas 512A-Dare shown initiating transmission (TX) of a common sinusoidal waveformof equal amplitude and frequency over each of the antennas starting atindividually staggered or offset times t_(b), t_(d), t_(a), t_(c)designed based on a prior composite sounding to synchronize the time ofarrival and the resultant phase of all signals at the RX antenna andadder 520. The composite sounding performed by the WAP and further theaggregation of the set of Linearly Independent partial soundingfeedback/responses allows the WAP to fully characterize the entirechannel including all paths between the four TX and the single RXantenna with a full beamforming matrix. The result is that there isconsiderable constructive interference between all four signal when theyarrive at the receiving stations antenna and summer 520. The aggregatesignal 542 at the output of the adder has increased in amplitude due tothe constructive interference with the other signals which collectivelyhave an additive effect on received signal strength. This maximizesreceived signal strength with the same transmit power, and is one of theattributes of effective beamforming.

FIG. 6 is a hardware block diagram of a MU-MIMO WAP 200 shown in FIGS.2A-B and FIG. 4B. The WAP supports either single sounding or compositesoundings depending on the relative capabilities of the WAP and thetarget station(s). The WAP in this embodiment of the invention isidentified as a 4×4 WAP supporting as many as 4 discrete communicationstreams over four antennas 680. The WAP couples to the Internet via anintegral wired interface 602 to a cable or digital subscriber line (DSL)modem 600. A packet bus 604 couples the modem to the MU-MIMO wirelesslocal area network (WLAN) stage 630. The WLAN stage includes a basebandmodule 632 and a radio frequency (RF) module 660 coupled to antennas680. In FIG. 6 only the transmit components of the baseband and RFportions of the WAP are shown. The WAP however has a full andcomplementary set of receive path components and operates fortransmitting and receiving communications from all associated wirelessstations on its network.

In the baseband portion 632 communications for each user/station areprocessed. In the embodiment shown two pairs of streams are beingprocessed for discrete MU delivery to two discrete stations/users. Thebaseband portion is dynamically configurable to support MU groups from2-4 users/stations. The communications “a, b” to the 1^(st) of the twousers are encoded and scrambled in encoder scrambler module 634A andde-multiplexed into two streams in demultiplexer 636A. Each stream “a,b” is subject to interleaving and constellation mapping in an associatedinterleaver mapper 638A and passed to the spatial mapper 640.Communications for the 2^(nd) user/station are encoded and scrambled inencoder scrambler module 634B and de-multiplexed into two streams indemultiplexer 636B. Each stream “c, d” is subject to interleaving andconstellation mapping in an associated interleaver mapper 638B andpassed to the spatial mapper 640. The spatial mapper maps the streamsonto each of the transmit chains using a full beamforming matrixprovided by sounding module 606 from a single or a composite sounding.The spatially multiplexed streams are injected into all OFDM tones650A-D of the four inverse discrete Fourier Transform (IDFT) modules652A-D respectively for Radio Frequency (RF) upconversion in RF stage660 and for transmission on the OFDM communication channel by each ofthe WAP's four antenna 680.

The RF stage includes 4 transmit chains each with its owndigital-to-analog converter (DAC) 662A-D, filter 664-D, upconverter668A-D and power amplifier 670A-D. Each of the four transmit chainscouples to a corresponding one of the WAP's four antenna 680. A commonoscillator 666 drives the upconverters 668A-D.

The WAP 601 also includes a sounding module 606 coupled to storage 620.The sounding module includes a station capability module 608, a soundingmode module 610, a sounding matrix generator 612, a sounding aggregator616, a gain normalizer, and a beamforming matrix expansion module 618.The station capability module 608 is configured to determine a soundingfeedback capability of the at least one targeted station node, in termsof a maximum number of streams. The station capability module couples tothe baseband portion of the WLAN stage to process packets received fromthe associated stations during a capabilities exchange portion of eachstation's WLAN association. These packet(s) contain information aboutthe stations capabilities, e.g.: bandwidth, number of streams supported,number of feedback streams supported, IEEE 802.11 standard support, etc.The sounding mode module 610 is configured to determine whether a numberof communication streams supported by the WAP matches the number ofcommunication streams contained in a sounding response from a targetedstation node, based on the capabilities determined by the stationcapability module. The sounding mode module then initiates either asingle sounding or a composite set of an integer “N” soundings of thecommunication channel between the WAP and the target station node basedon an affirmative and a negative match determination respectively. Thesounding matrix generator 612 is coupled to the sounding mode module andto the spatial mapper 640. It is configured to generate a singlesounding spatially mapped by the spatial mapper 640 using acorresponding spatial mapping matrix in response to an affirmative matchdetermination by the sounding mode module. Alternately, the soundingmatrix generator is configured to generate “N” linearly independent (LI)spatial mapping matrices (SMM) for sequential injection into the spatialmapper 640 with each associated with a corresponding one of thesoundings in the composite set of soundings from the WAP, responsive toa negative match determination by the sounding mode module. In anembodiment of the invention the sounding matrix generator generates the“N” LI SMM using a lookup table. In another embodiment of the inventionthe sounding matrix generator generates the “N” LI SMM using a trial anderror method or via direct calculation. In another embodiment of theinvention the sounding matrix generator generates at least one referenceSMM, e.g. reference SMM 643, fixed across soundings for spatiallymapping one or more reference tones, e.g. reference tones 651D, 651Hwith fixed indices across soundings in the composite set of soundingsand the LI SMM, e.g. LI SMM 642 for mapping remaining tones in each ofthe composite set of soundings. The sounding aggregator 616 couples tothe baseband portion of the WLAN stage and is configured to aggregate aplurality of partial sounding feedback matrices received from the atleast one targeted station node in response to the composite soundingsgenerated by the sounding matrix generator. These are stored 622 instorage 620. The gain normalizer 617 couples to the sounding aggregatorand is configured to normalize any differences in either or bothamplitude and phase of the sequential partial sounding feedbackmatrices. The gain normalizer determines any differences in amplitudeand or phase in the sounding feedback for those tones designated asreference tones and applies the resulting normalizing scale factors tothe sounding feedback matrices for all remaining tones in each set oftones associated with each sounding feedback. The beamforming expansionmodule 618 is configured to expand the composite set of partialnormalized sounding feedback matrices aggregated by the soundingaggregator and normalized by the gain normalizer into a full beamformingmatrix for spatially mapping downlink communications between the WAP andthe either a single station (MIMO) or two or more stations (MU-MIMO);thereby enhancing the communication capabilities of the WAP

In an embodiment of the invention the sounding module and associatedmodules thereof may be implemented by a processor integrated with theWAP and executing program code 626 stored in memory element 620.

FIG. 7 is a process flow diagram of processes associated with enhancedsounding capabilities of a MU-MIMO WAP in accordance with an embodimentof the current invention. Processing begins at process block 700 inwhich a capabilities exchange between the WAP and at least one targetedstation is initiated. In this capabilities exchange the communicationcapabilities of the station are determined such as: number of datastreams supported, number of antenna; number of streams in soundingresponse or feedback; the modulation and coding schema (MCS) andsupported bandwidth. Next in process 702 the number of symbols in thesounding field, e.g. the VHT-LTF field, are determined for each tonebased on the number of streams supported in the explicit soundingfeedback from the station. Next in process 704 the building of thesymbol(s) for the sounding field is initiated, including the proscribedbits for each tone, the proscribed phase and phase change for eachsymbol of the sounding field and streams within each sounding symbol, asper the IEEE 802.11ac or other relevant standard. Then in process 706 adetermination is initiated as to whether the number of communicationstreams supported by the WAP matches the number of streams for which thestation can characterize the channel in the sounding feedback orresponse from the station. In decision process 710 an affirmative matchdetermination that the supported streams in the station's soundingfeedback matches the # of streams supported by the WAP results in thepassing of control to the single sounding branch of processes commencingwith process 720. Alternately if a negative match determination is made,that the number of supported streams in the station's sounding feedbackis less than the number of streams supported by the WAP then control ispassed to the composite sounding branch of processes commencing withprocess 740.

The single sounding branch or processes 720-728 utilize the matchedsounding capabilities of the WAP and station to obtain from the stationa full beamforming matrix for spatially mapping subsequentcommunications. In the initial single sounding process 720 each soundingstream on the WAP is spatially mapped using a fixed spatial mappingmatrix (SMM) e.g. an identity matrix identity matrix, to the transmitantennas of the WAP. Then in process 722 a single Explicit soundingpacket is transmitted to the target station. The sounding packetincludes a number of symbols corresponding to the number of feedbackstreams supported by the station in the sounding response. Only a singlesounding is required because of the match between the WAP and stationsstream sounding capabilities. Next control is passed to process 724 inwhich the response to the sounding is received by the WAP. That responseincludes the “V” and Σ matrices in accordance with the IEEE 802.11acstandard, or the “H” matrices in accordance with the IEEE 802.11nstandard, for example. Where the single sounding feedback conforms withIEEE 802.11ac standard, the full beamforming matrix “V” obtained in thesingle sounding feedback from the station is used in process 728 tocontrol the spatial mapping of subsequent downlink communications ontoeach of the WAP's antenna. Control then returns to process 700.

The composite sounding branch or processes 740-754 utilize compositesoundings to overcome the lack of support in the station for soundingswhich exploit the full capability of the WAP. The station is induced tosend partial soundings responses to the WAP which collectively allow theWAP, by virtue of their linear independence from one another, to expandthe partial matrices to a full beamforming matrix thereby allowing theWAP to exploit its full communication capabilities for subsequentdownlink communications with the station(s). In the initial compositesounding process 740 a determination is made as to the number “N” ofsuccessive sounding packets required to fully characterize thecommunication channel between the WAP and the station(s). Next inprocess 741 the reference tone(s) that will be used to normalizedifferences between successive “N” ones of the composite soundings areidentified. Next in process 742 a determination is made as to a set oflinearly independent (LI) spatial mapping matrices (SMM) with one memberof each set spatially mapping the sounded streams onto each of the WAP'sMIMO antenna. These LI SMM will be used for mapping all of the OFDMtones in the communication channel being sounded except for those tonesselected as reference tones. Next in process 743 at least one fixedreference SMM, a.k.a. “Q_(ref)” is identified for spatially mapping theselected/identified reference tones. In decision process 744 adetermination is made as to whether there is a remaining one of thesuccessive sounding packets to be sent. If there are not, then controlpassed to process 751. Alternately, if there is a remaining compositesounding to be made then control passes to process 746. In process 746the next linearly independent spatial mapping matrix is used to map thecurrent sounding, specifically the mapping of each sounding symbol andassociated streams to one or more of the WAPs transmit antenna for alltones except the reference tones. The reference tones are spatiallymapped with the fixed SMM. Reference tones have the same indices, e.g.center frequency, across the successive partial soundings. The referencetone(s) are spatially modulated with a reference spatial mapping matrix(SMM), e.g. SMM 343. Where there is more than one reference tone perpartial sounding there may be more than one reference SMM, however, fora reference tone of a given index, the same reference SMM is used acrossthe successive partial soundings. Then in process 748 the next linearlyindependent explicit sounding packet is transmitted from the WAP to thestation(s). The sounding packet includes a number of symbolscorresponding to the number of feedback streams supported by the stationin the sounding response. Next in process 750 the next partial soundingfeedback matrices received from the station in response to the explicitsounding are aggregated with the sounding responses from the station tothe other soundings in the composite set. Control then returns todecision process 744. When all composite soundings have been sent, andall sounding responses responsive thereto have been aggregated controlpasses to process 751.

Next in process 751 the aggregated partial sounding feedback matricesfor the reference tones are used to normalize stored explicit partialsounding feedback matrices for all remaining tones. Specifically, theWAP normalizes differences in amplitude and or phase between successiveones of the composite soundings by measuring relative differences in thesounding feedback for those tones designated as reference tones andapplying the resulting normalizing scale factors to the partial soundingfeedback matrices for all remaining tones in each set of tonesassociated with each sounding feedback. In process 752 the aggregatedstored normalized explicit partial sounding feedback matrices from thestation, are used by the WAP to derive a full beamforming matrix. Thismatrix expansion allows the WAP to derive a full beamforming matrixcompletely characterizing the communication channel between each of theWAP's antennas and the target station(s) receive antennas. Then inprocess, 754 the full beamforming matrix derived on the WAP from thenormalized partial soundings of the station are used to spatially mapdownlink data onto each of the WAP's antenna for transmission ofsubsequent downlink communications therewith. Control then returns toprocess 700.

The components and processes disclosed herein may be implemented insoftware, hardware, firmware, or a combination thereof including programcode software, a memory element for storing the program code softwareand a processor for executing the program code software, withoutdeparting from the scope of the Claimed Invention.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. A wireless access point (WAP) apparatus having aplurality of antennas and supporting multiple-input multiple-output(MIMO) wireless communications with associated station nodes on aselected one of a plurality of orthogonal frequency division multiplexed(OFDM) communication channels each including a plurality of tones; andthe wireless access point apparatus comprising: a sounding mode moduleconfigured to determine whether a number of communication streamssupported by the WAP matches the number of communication streamscontained in a sounding response from at least one targeted station nodeamong the associated station nodes, and initiating either a singlesounding or a composite set of an integer “N” soundings of thecommunication channel between the WAP and the at least one targetedstation node based on an affirmative and a negative match determinationrespectively; a sounding matrix generator configured to generate “N”linearly independent (LI) spatial mapping matrices (SMM) each associatedwith a corresponding one of the soundings in the composite set ofsoundings from the WAP, together with at least one reference SMM forspatially mapping at least one reference tone across all of thesoundings in the composite set of soundings, responsive to a negativematch determination by the sounding mode module; a sounding aggregatorconfigured to aggregate a plurality of partial sounding feedbackmatrices received from the at least one targeted station node inresponse to the composite soundings generated by the sounding matrixgenerator; a gain normalizer to normalize the partial sounding feedbackmatrices aggregated by the sounding aggregator utilizing the referenceSMM; and a beamforming expansion module configured to expand theplurality of normalized partial sounding feedback matrices normalized bythe gain normalizer into a full beamforming matrix for spatially mappingdownlink communications between the WAP and the at least one targetedstation node; thereby enhancing the communication capabilities of theWAP.
 2. The WAP apparatus of claim 1, further comprising: a spatialmapper for spatially mapping selected ones of the tones among theplurality of tones in the communication channel with the at least onereference SMM, and remaining ones of the tones with the LI SMM.
 3. TheWAP apparatus of claim 1, further comprising: the gain normalizerfurther configured to normalize the partial sounding feedback matricesof each sounding in the composite set of soundings relative to eachother; utilizing the partial sounding feedback matrices associated withthe reference SMM.
 4. The WAP apparatus of claim 1, further comprising:the gain normalizer configured to normalize an amplitude of the partialsounding feedback matrices across the composite set of soundings basedon differences in amplitude of the partial sounding feedback matricesassociated with the reference SMM.
 5. The WAP apparatus of claim 1,further comprising: the gain normalizer configured to normalize a phaseof the partial sounding feedback matrices across the composite set ofsoundings based on differences in the phase of the partial soundingfeedback matrices associated with the reference SMM.
 6. The WAPapparatus of claim 1, further comprising: the sounding matrix generatorfurther configured to generate the composite set of soundingssequentially in discrete sounding packets, and the sounding in eachpacket spatially mapped to associated ones of the WAP's antennas with acorresponding one of the “N” linearly independent SMM.
 7. The WAPapparatus of claim 1, further comprising: the sounding matrix generatorfurther configured to generate the composite set of soundingssequentially in discrete symbols within a single sounding packet, andthe sounding in each symbol spatially mapped to associated ones of theWAP's antennas with a corresponding one of the “N” linearly independentSMM.
 8. The WAP apparatus of claim 1, further comprising: the soundingmode module further configured to initiate the composite set ofsoundings of the communication channels between the WAP and at least twomulti-user (MU) MIMO targeted station nodes.
 9. The WAP apparatus ofclaim 1, wherein the plurality of partial sounding feedback matricesaggregated by the sounding aggregator comprise at least one of: partialbeamforming matrices “V”, signal-to-noise (SNR) matrices “SNR”, andpartial channel matrices “H”.
 10. A method for operating a wirelessaccess point (WAP) having a plurality of antennas and supportingmultiple-input multiple-output (MIMO) wireless communications withassociated station nodes on a selected one of a plurality of orthogonalfrequency division multiplexed (OFDM) communication channels eachincluding a plurality of tones; and the method comprising: determiningwhether a number of communication streams supported by the WAP matchesthe number of communication streams characterized in a sounding responsefrom at least one targeted station node among the associated stationnodes; initiating either a single sounding or a composite set of aninteger “N” soundings of the communication channel between the WAP andthe at least one targeted station node based on an affirmative and anegative match determination respectively, in the determining act;generating “N” linearly independent (LI) spatial mapping matrices (SMM)each associated with a corresponding one of the soundings in thecomposite set of soundings from the WAP, responsive to a negative matchdetermination in the determining act; generating at least one referenceSMM for spatially mapping at least one reference tone across all of thesoundings in the composite set of soundings, responsive to a negativematch determination in the determining act; aggregating a plurality ofpartial sounding feedback matrices received from the targeted one of theassociated station nodes in response to the composite soundingsgenerated in the generating act; normalizing the partial soundingfeedback matrices aggregated in the aggregating act, utilizing thereference SMM; and expanding the plurality of partial sounding feedbackmatrices normalized in the normalizing act into a full beamformingmatrix for spatially mapping downlink communications between the WAP andthe targeted one of the associated station nodes; thereby enhancing thecommunication capabilities of the WAP.
 11. The WAP apparatus of claim10, further comprising: spatially mapping selected ones of the tonesamong the plurality of tones in the communication channel with the atleast one reference SMM, and remaining ones of the tones with the LISMM.
 12. The WAP apparatus of claim 10, further comprising: normalizingthe partial sounding feedback matrices of each sounding in the compositeset of soundings relative to each other; utilizing the partial soundingfeedback matrices associated with the reference SMM.
 13. The WAPapparatus of claim 10, further comprising: normalizing an amplitude ofthe partial sounding feedback matrices across the composite set ofsoundings based on differences in amplitude of the partial soundingfeedback matrices associated with the reference SMM.
 14. The WAPapparatus of claim 10, further comprising: normalizing a phase of thepartial sounding feedback matrices across the composite set of soundingsbased on differences in the phase of the partial sounding feedbackmatrices associated with the reference SMM.
 15. The method for operatinga WAP of claim 10, wherein the generating act further comprises:generating the composite set of soundings sequentially in discretesounding packets; spatially mapping the sounding in each packet toassociated ones of the WAP's antennas with a corresponding one of the“N” linearly independent SMM.
 16. The method for operating a WAP ofclaim 10, wherein the generating act further comprises: generating thecomposite set of soundings sequentially in discrete symbols within asingle sounding packet; and spatially mapping the sounding in eachsymbol to associated ones of the WAP's antennas with a corresponding oneof the “N” linearly independent SMM.
 17. The method for operating a WAPof claim 10, further comprising: initiating the composite set ofsoundings of the communication channels between the WAP and at least twomulti-user (MU) MIMO targeted station nodes.
 18. The WAP apparatus ofclaim 10, wherein the plurality of partial sounding feedback matricesaggregated in the aggregating act comprise at least one of: partialbeamforming matrices “V”, signal-to-noise (SNR) matrices “SNR”, andpartial channel matrices “H”.